Glue for form ply for sp10 plinth?

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Alec124c41
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Re: Glue for form ply for sp10 plinth?

Post by Alec124c41 » 25 Jun 2014 04:56

Use it for the top, sanded, blown clean, and filled with liquid epoxy to make a smooth surface. That's CHARACTER!
Congratulations on the match.

Cheers,
Alec.

cafe latte
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Re: Glue for form ply for sp10 plinth?

Post by cafe latte » 25 Jun 2014 07:34

Alec124c41 wrote:Use it for the top, sanded, blown clean, and filled with liquid epoxy to make a smooth surface. That's CHARACTER!
Congratulations on the match.

Cheers,
Alec.
Thanks Alec,
Hmm that might be an idea... :)
Regards
Chris

AudioFeline
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Re: Glue for form ply for sp10 plinth?

Post by AudioFeline » 02 Dec 2019 12:59

cats squirrel wrote:
13 Jun 2014 00:29
...one of the tendencies people have with ply is to use multiple layers, so that the plinth ends up about 100mm thick. At first glance, this would seem to meet the mass and even stiffness criteria, but damping is woeful. And as plywood has a critical frequency of 16kHz for 1mm, a 100mm thick plinth will have a critical frequency of 16000/100, which equals 160Hz. As the plinth will probably resonate at a frequency above this, no amount of extra weight(mass) is going to make a jot of difference....
So cats, if more layers of ply in a plinth is counter-productive, what would be a suggested thickness for a resonate freq not to be a problem?

cats squirrel
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Re: Glue for form ply for sp10 plinth?

Post by cats squirrel » 02 Dec 2019 15:51

well, as the Irishman said (BTW, I love the Irish!) when asked the way to Dublin (when in Cork) "I wouldn't start from here!"

Problem with ply is its very poor at damping. Woefully so. I've just been working on some numbers on another thread, and you may be interested and surprised to find that an 18mm thick plinth of ply is much better than 100mm of ply, due mostly to the thicker ply's stiffness. Losses for the 100mm ply is 6 to 12dB (20Hz to ~1kHz) but for the 18mm ply (same perimeter measurements) is 14 to 26dB, same frequencies.

If one kept with all the same parameters of ply, but increase damping factor from 0.04 to 0.4, then losses would 30 - 36dB over the same frequency range. Therefore, I wouldn't use plain ply for a plinth.

Why do people use ply? Because it is relatively cheap and easily obtainable. Contrast that with materials fit for purpose, like Panzerhloz, Permali, Jarrah, Ironbark and some Ironwoods, and not forgetting isophthalic polyester resin/bentonite clay granules, and you can see why ply (at least in the first instance) seems so attractive, 'it's only a bloody plinth, after all!' :?

I must add that I think the Technics SP range to be very good wrt vibrations, or lack of them. However, there are problems with the other parts of the turntable, like the cart/arm vibrations, and use of a metal platter mat. Many audiophiles will be using a MC cart and heavy arm, which together can generate a lot of vibration. Most of those arms are mounted in unsuitable arm boards/plinths. Also, as the (vinyl) record picks up a lot of vibration from the 'speakers, if using a metal platter mat, these vibrations can be transferred to the TT, then into the plinth, or more likely, bounced back to the record, for intermodulation to take place.

cafe latte
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Re: Glue for form ply for sp10 plinth?

Post by cafe latte » 02 Dec 2019 23:10

cats squirrel wrote:
02 Dec 2019 15:51
well, as the Irishman said (BTW, I love the Irish!) when asked the way to Dublin (when in Cork) "I wouldn't start from here!"

Problem with ply is its very poor at damping. Woefully so. I've just been working on some numbers on another thread, and you may be interested and surprised to find that an 18mm thick plinth of ply is much better than 100mm of ply, due mostly to the thicker ply's stiffness. Losses for the 100mm ply is 6 to 12dB (20Hz to ~1kHz) but for the 18mm ply (same perimeter measurements) is 14 to 26dB, same frequencies.

If one kept with all the same parameters of ply, but increase damping factor from 0.04 to 0.4, then losses would 30 - 36dB over the same frequency range. Therefore, I wouldn't use plain ply for a plinth.

Why do people use ply? Because it is relatively cheap and easily obtainable. Contrast that with materials fit for purpose, like Panzerhloz, Permali, Jarrah, Ironbark and some Ironwoods, and not forgetting isophthalic polyester resin/bentonite clay granules, and you can see why ply (at least in the first instance) seems so attractive, 'it's only a bloody plinth, after all!' :?

I must add that I think the Technics SP range to be very good wrt vibrations, or lack of them. However, there are problems with the other parts of the turntable, like the cart/arm vibrations, and use of a metal platter mat. Many audiophiles will be using a MC cart and heavy arm, which together can generate a lot of vibration. Most of those arms are mounted in unsuitable arm boards/plinths. Also, as the (vinyl) record picks up a lot of vibration from the 'speakers, if using a metal platter mat, these vibrations can be transferred to the TT, then into the plinth, or more likely, bounced back to the record, for intermodulation to take place.
Form ply here in Aus is hardwood and very dense and heavy. I am sure layers of form ply will be better than one layer. My Commonwealth plinth is 45kg not ply but wood, on the long shelf it was the only turntable that did not suffer from the vibrating shelf which was because of the massive plinth so big for sure has damping advantages.
Chris

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Re: Glue for form ply for sp10 plinth?

Post by cats squirrel » 02 Dec 2019 23:45

mass doesn't damp!

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Re: Glue for form ply for sp10 plinth?

Post by cafe latte » 02 Dec 2019 23:51

cats squirrel wrote:
02 Dec 2019 23:45
mass doesn't damp!
No but the same energy is in more material so it does in effect and each layer of material damping is beneficial too as resonant frequency is different. Energy can not be created or destroyed it goes to another form. L arge massy plinth is a good thing and I have proved it in less than ideal conditions on the long shelf. Lighter plinths vibrations made it back to the cart but not on the Commonwealth plinth.
Chris

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Re: Glue for form ply for sp10 plinth?

Post by cats squirrel » 03 Dec 2019 02:18

no it doesn't. More layers (or thicker plinth) has a much higher flexural rigidity. For ordinary birch ply, an 18mm thick panel (plate) has a static flexural rigidity of 783, whereas a 100mm plate has a static flexural rigidity of 134254 ! This is the reason why the losses are so poor.

I haven't tested hardwood ply yet (I have some, but from African hardwoods) but tests on most hardwoods show very little damping. However, if your ply is made from iron bark or iron woods (or Jarrah) then it may be very good at damping. However, thin will work, thick will not be as good.

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Re: Glue for form ply for sp10 plinth?

Post by cafe latte » 03 Dec 2019 05:59

cats squirrel wrote:
03 Dec 2019 02:18
no it doesn't. More layers (or thicker plinth) has a much higher flexural rigidity. For ordinary birch ply, an 18mm thick panel (plate) has a static flexural rigidity of 783, whereas a 100mm plate has a static flexural rigidity of 134254 ! This is the reason why the losses are so poor.

I haven't tested hardwood ply yet (I have some, but from African hardwoods) but tests on most hardwoods show very little damping. However, if your ply is made from iron bark or iron woods (or Jarrah) then it may be very good at damping. However, thin will work, thick will not be as good.
The form ply is very heavy and red hardwoods with a lot of resin or epoxy. It is designed for concrete form work and made to be water proof. IMO is would make an excellent plinth if anti adhesive coat is removed a plinth as good as the Commonwealth could be constructed. Dont know about your flexural rigidity, all I know is more mass means energy is trying to more more molecules and more vibrational energy converted to harmless heat. The proof is in the pudding so to speak ie the Commonwealth plinth.
Chris

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Re: Glue for form ply for sp10 plinth?

Post by totem » 03 Dec 2019 23:00

I used a combination of Bamboo cabinet grade ply and MDF for this plinth and Bamboo
laminated ply for the 3 arm boards. I found this particular Bamboo to be hard on cutters but very stable.
The armboards are mounted using threaded inserts epoxied in place with SS counter sunk fasteners.
IMG_3180_Fotor.jpg
(174.89 KiB) Downloaded 28 times

totem
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Re: Glue for form ply for sp10 plinth?

Post by totem » 03 Dec 2019 23:01

Meant to note the TT is a Victor TT-101.

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Re: Glue for form ply for sp10 plinth?

Post by cafe latte » 04 Dec 2019 20:14

totem wrote:
03 Dec 2019 23:00
I used a combination of Bamboo cabinet grade ply and MDF for this plinth and Bamboo
laminated ply for the 3 arm boards. I found this particular Bamboo to be hard on cutters but very stable.
The armboards are mounted using threaded inserts epoxied in place with SS counter sunk fasteners.
IMG_3180_Fotor.jpg
=D> Very nice indeed!!
Chris